Radiowave imaging research (RIM) for determining the electrical conductivity of the rock in borehole section OL-KR4−OL-KR10 at Olkiluoto, Finland
Arto Korpisalo 1 3 Eero Heikkinen 21 Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland.
2 Pöyry Finland Oy, Jaakonkatu 2, FI-01620 Vantaa, Finland.
3 Corresponding author. Email: arto.korpisalo@gtk.fi
Exploration Geophysics 46(2) 141-152 https://doi.org/10.1071/EG13057
Submitted: 17 June 2013 Accepted: 23 March 2014 Published: 14 May 2014
Abstract
We carried out an electromagnetic cross-borehole survey as a feasibility target for the radiowave imaging method (RIM) in a possible area for the repository of spent nuclear fuel in Finland to reveal the properties of granitic, i.e. crystalline bedrock. The resistivity in the bedrock varies strongly in the range of tens to tens of thousands of ohm metres due to sulphide bearings. Because increased electrical conductivity is often associated with rock mass deformation (clay and water-bearing fractures, sulphide- and graphite-bearing zones), the obtained information could be used, for example, in determining the integrity of the rock mass.
This paper describes the field work, as well as the interpretation and comparison of the results of radiowave imaging with other geophysical results in one borehole section at the 200–600 m depth level from 2005. The results verified that the method can be used to delineate and follow sulphide-bearing horizons between boreholes. The detected low and high resistivity zones and their apparent shapes and orientations are in fair agreement with geological and other geophysical results. The material properties differ from those recorded in seismic tomography from the same location, although the reflections partly describe the boundaries of domains differing in electrical properties. Using borehole geometry, similar but more detailed information is obtained than with electric and electromagnetic measurements from the ground surface. Our first experience with RIM convinced us that it can also be utilised in such challenging environments as at Olkiluoto, and not only when massive deposits exist.
Key words: attenuation, cross-borehole, EMRE, Olkiluoto, RIM, tomography.
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